In Japan, a more specific release date of September 15 has been previously announced.
Here is an overview of the game, via its Steam page:
Welcome to UNDERTALE. In this RPG, you control a human who falls underground into the world of monsters. Now you must find your way out… or stay trapped forever.
((Healthy Dog’s Warning: Game contains imagery that may be harmful to players with photosensitive epilepsy or similar condition.))
- Killing is unnecessary: negotiate out of danger using the unique battle system.
- Time your attacks for extra damage, then dodge enemy attacks in a style reminiscent of top-down shooters.
- Original art and soundtrack brimming with personality.
- Soulful, character-rich story with an emphasis on humor.
- Created mostly by one person.
- Become friends with all of the bosses!
- At least 5 dogs.
- You can date a skeleton.
- Hmmm… now there are 6 dogs…?
- Maybe you won’t want to date the skeleton.
- I thought I found a 7th dog, but it was actually just the 3rd dog.
- If you play this game, can you count the dogs for me…? I’m not good at it.